This invention relates to the field of processing signature-based payment transactions wherein a merchant's records of payment are generated and maintained electronically without generation of paper records while providing paper receipts to customers as needed. It relates more particularly to a point of sale system using a transparent digitizer positioned over a display and still more particularly to a method of eliminating the effects of skew between the digitizer and the display.
In retailing and similar areas, the volume of transactions is often such that management of paper records is objectionably burdensome. Therefore paper records are being replaced by digital storage media wherever feasible. However, digital storage devices have not been able to eliminate the need for paper storage in many financial transactions requiring verification by a human signature. In such transactions paper documentation has continued to be the norm. This is particularly true in retail credit purchases wherein a signed copy of a sales receipt is required for validating the transfer of funds or for resolving disputes concerning the purchase. These paper records present inefficiency in retrieval as well as in storage.
It has now been found that signature verification of a credit transaction can be achieved without storing paper records, if a digitized copy of the obligor's signature is maintained along with a digital record of the sale. The present invention finds particular application in a newly proposed system which assembles all of the essential information regarding a sale, accepts and digitizes the purchaser's signature, prints a signed sales receipt for the purchaser, and stores a complete electronic record of the transaction, all in one operation at the point of sale station. The proposed arrangement, as discussed in commonly owned, copending application Ser. No. 575,096, filed Aug. 30, 1990, the disclosure of which is incorporated herein by reference, utilizes a transparent digitizer equipped with a stylus. When a user moves the stylus across the front surface of the digitizer, the position of the stylus is sensed and digitized. The digitizer is mounted against the surface of a liquid crystal display which is activated to display information relating to a transaction then in progress. Information which is so displayed can be seen through the digitizer and therefore is visible to the user.
In the above described system the digitized coordinates of the stylus are utilized to cause the display device to display a corresponding mark. It is desired that the displayed mark be in exact registration with the stylus, so that the user can make a normal handwritten signature and see it displayed as the writing progresses However, this requires that the digitizer and the display be in precise alignment. Any misalignment between the digitizer and the display device causes the progressing signature display to be out of registration with the movement of the stylus, thereby complicating eye-hand coordination for the user. A misalignment condition also produces a unsightly receipt by causing the signature to be printed out of alignment with other transaction information.
Misalignment consists of three components: offset, scale and skew. Offset is caused by a misalignment of absolute origins between the display and the digitizer coordinate systems. Scale misalignment is caused by an unequal distance per coordinate count between the two coordinate systems. Skew is caused by an apparent rotation between the two coordinate systems. Offset and scale correction is a simple matter while skew correction is not. This invention is an improvement over other methods in that it takes this skew factor into account.
Skew may be caused by inaccurate mounting of the digitizer on the display. It may also result from uneven heating of the digitizer surface during use. Digitizers which employ a resistive coating on the active surface are particularly susceptible to heat-induced skew. Other causes of skew may be low quality electronic components or a non uniformly deposited resistive coating on the digitizer. This invention addresses such problems.